Recently, an oxide film constituted by SiO2 having a film thickness of about 50 through 100 nm installed in a thin film transistor on a glass or plastic substrate is mainly used in a liquid crystal display or a flexible display. In a case of the liquid crystal display, due to a restriction of a thermal durable temperature of an inexpensive glass substrate (for example, non-alkaline glass), a manufacturing process demands, under the current circumstance, about 300° C. or lower. On the other hand, in a case of the flexible display, it is general practice that a process (transcription method) in which the TFT device is once prepared on the glass substrate and transcribed onto the plastic substrate. Hence, similarly, the process of 300° C. or lower is demanded. In order to aim, hereafter, at a simplification of the manufacturing process, it is expected that a technique of preparing directly a silicon insulating film and an insulating film on the plastic substrate is needed. In this case, a process under 200° C. or lower heat durable temperature of a micromolecular film such as a polyethersulfone (PES) is demanded. A CVD using mainly plasma (PE-CVD: Plasma-Enhanced Chemical Vapor Deposition) has been used in a preparation of an insulating film at about 300° C. However, in order to apply this PE-CVD to the next generation technology, there are problems of requiring an improvement in an interface characteristic to a silicon-series thin film to make deviations in threshold voltages small, requiring an anneal process for about two hours at about 600° C. to make a characteristic improvement after the formation of the film, a difficulty in a thin-film formation for higher performance, increasing remarkably by one digit a carbon impurity concentration within a film with a film formation temperature reduced hereafter to 200° C. and so forth (non-patent document 1).
It is necessary for the next generation TFT to realize a transistor characteristic which has a lower threshold voltage with a high electron mobility and is stable as a device. This depends on how high-quality gate insulating film is achieved. Recently, a main aim has been placed on (1) in what way a plasma source of high-density and low-damage should be developed in the plasma CVD process: (2) a development in a formation technique of a gate insulating film which does not use plasma at all: and (3) a development of a technique of maintaining a clear interface with a silicon film which provides a substrate.
Then, from a viewpoint of (2), especially with a fact that the damage in the interface cannot be avoided taken into consideration, an apparatus to which a catalytic chemical vapor deposition method for the preparation of the gate insulating film has been applied has been developed. This apparatus is perfectly plasma-free and can correspond to a large area by arranging a high-melting point metallic converter on a large area and has a feature that a gas utilization efficiency is high and a usage quantity of gas can be suppressed (non-patent document 2).
In addition, from a standpoint of (3), if an SiO2 deposition film prepared during the plasma CVD process is used in the gate insulating film, impurities on a silicon surface are directly introduced onto the interface. Hence, from the standpoint of large deviations of values of interface level densities, double-layer structured gate oxidation films have been prepared. First, the insulating film of a first layer is formed under the low temperature heat oxidation. A thermal oxidation rate is very low at a low temperature equal to or lower than 300° C. so that either a plasma-oxidation or a photo-oxidation using plasma or light has been used in place of heat. In order to secure a dielectric strength voltage and to realize a low leakage current, a second insulating layer (upper layer) is formed by means of a high-density, high-frequency plasma CVD method which does not give damage against the first layer oxidation film and against the interface. In a case where the oxidation film of the first layer is not used, the interface level density is 1×1011 [cm2/eV]. However, if the photo-oxidation film of 2 nm is prepared, the interface level density is reduced to 4×1010 [cm2/eV]. A treatment time for about 2 minutes is needed, in the case of the photo-oxidation film, for a ten-minute plasma oxidation at 300° C. to prepare the heat oxidation film of 2 nm. It is expected that it needs a further treatment time if both processes are carried out at 200° C. Hence, in a case where the double-layer structure is adopted, it is necessary to suppress a reduction in a throughput to a minimum limit.
On the other hand, a method of accumulating SiO2 film in proximity to room temperature is disclosed by performing an ultraviolet photo irradiation using an Excimer Radiation System under O2 atmosphere including a raw material gas of an organosilicon (patent document 1). According to this method, a binding energy of C—H, C—C, and so forth in most organic-series material gas such as TEOS (Tetraethoxysilane) is equal to or below 6 eV. Hence, the binding of the raw material gas can easily be broken at a normal temperature and an oxide film (SiO2) having no thermal nor plasma damage can be formed.
It should be noted that, in a case where the formation of SiO2 film onto the silicon thin film on the plastic or glass substrate is applied, an irradiation of ultraviolet light is carried out toward the substrate. Thus, photons not absorbed in a gaseous phase are irradiated onto a substrate material over the silicon thin film. For example, OA-10 (manufactured by Nippon Electric Glass) which is a representative low-temperature polysilicon thin film transistor substrate has a light absorption having a shorter wavelength than 250 nm. Hence, a light damage is introduced to the substrate. Then, there is a possibility that a problem of a reduction in a tight attachment between the substrate and the silicon thin film due to the light irradiation is generated. This can be applied equally well to the plastic whose absorption for ultraviolet light is larger than the glass and whose light degradation is severer than the glass.
The application of TEOS ozone CVD technique in which a preparation of an inter-layer insulating film which is bond-free and has a favorable step coverage to the thin film transistor insulating film has been discussed. However, it is under the present situation that a sufficient film formation speed is not obtained at a process temperature of 300° C. or lower and it lacks in practicality (non-patent document 4). This may because a reactivity of ozone molecules is reduced at a temperature of 300° C. or lower, namely, a reaction probability of decomposition reaction from ozone to a generation of oxygen atoms shown in a reaction equation described below is reduced so that ozone cannot function as a decomposition agent or an oxidation agent of TEOS.O3→O2(3Σ)+O(3P)
As far as a film quality is concerned, it is reported that hydrogen and carbon are left in the film as impurities in the same way as the film formed by the plasma CVD at 300° C. or lower so that the film becomes porous and provides a film whose etch resistance has been degraded and whose relative permittivity is reduced from an ideal value (3.9) and having an insulating characteristic of large leakage current (non-patent document 5 and non-patent document 6)    Non-patent document 1: Sharp technical report, by Yukihiko Nakata et al, 80, 31 (2001).    Non-patent document 2: “Applied physics” vol. 73, No. 7, pp. 0935-0938 (2004) by Ozono Shuji et al.    Non-patent document 3: “Vacuum” of Japan Vacuum Society magazine, 47, 5, pp. 357 (2004) by Yukihiko Nakata et al    Patent document 1: Japanese Patent Application First Publication No. 2001-274155.    Non-patent document 4: Shareef et al., J. Vac. Sci. Techol. B14, 744 (1996).    Non-patent document 5: A. M. Nguyen, J. Vac. Sci. Technol. B8533 (1999).    Non-patent document 6: H. U. Kim, and S. W. Rhee, J. Electrochem. Soc. 147 (2000)1473.